Morphopathological and molecular morphometric characterization of Waitea circinata var. prodigus causing a novel sheath spot disease of maize in India.

Maize brown sheath spot (MBSS), a new disease of maize, was discovered while surveying for maize leaf and sheath blight diseases in the Indian states of Assam, Jharkhand, Meghalaya, Manipur, and Odisha. Maize is the third most important cereal after rice and wheat in India. Unlike banded leaf and sheath blight (BLSB) disease caused by Rhizoctonia solani, MBSS symptoms on maize were discrete and limited to sheaths only. Symptoms of MBSS in the field were initially water soaked necrotic lesions of 1 to 2 cm in diameter on the lowermost leaf sheaths, which then progressed to the upper sheaths. Lesions coalesced and covered approximately 2 to 5% of the sheath area. Infected dried lower leaves were shed while infected upper leaves remained on the stem. The pathogen was isolated, characterized morphologically, pathologically, and molecularly, and identified as Waitea circinata var. prodigus (Wcp); a basidiomycete known to cause basal leaf blight of seashore paspalum. The internal transcribed spacer sequence 2 (ITS2) of rDNA from MBSS isolates formed a well-supported clade with known Wcp isolates. Molecular morphometric analysis of the ITS2 regions of the five known varieties of W. circinata detected distinguishing variations in GC content, compensatory base changes (CBCs), hemi-compensatory base changes (hCBCs), indels, and altered base-pairing of helices. Variation in these characteristics may indicate that varieties are distinct biological species within W. circinata sensu lato. The geographical distribution and potential impacts of MBSS on the maize crop in India necessitates further investigations on pathogen identification and disease management.